Manufacturers have not anticipated that vermin invade an electronics apparatus and lower the reliability. Thus no positive countermeasure have been taken against the vermin, and such environment results in publicizing no information about failures, troubles or abrupt malfunctions of electronics apparatuses due to carcasses or droppings of the vermin. As a result, only a few ideas have been adopted in the countermeasure against the vermin, such as, a housing shape of an electronics apparatus is changed, molding material or paint including repellent is placed or painted on an invading route. However, those measures are not compatible with downsize of the electronics apparatuses, and applying the paint including the repellent to stuffed printed wiring boards possibly incurs lowering the insulation of the electronics circuits. Therefore those measures have not prevailed in the industry.
A technique of mixing repellent directly to ink material such as solder-resist was introduced as a method of producing material, which included the repellent, of electronics components. The printed wiring board, employing this conventional material that includes the repellent, cannot maintain the vermin repelling effect for a long period because an amount of the repellent decreases substantially (vaporization, decomposition, polymerization) due to heating for soldering parts to mount. To overcome this inconvenience, an amount of the repellent corresponding to the amount decreased should be mixed additionally. The greater amount of repellent lowers film-forming property (printability, hardness of film surface, adherence), increases the cost, and does not suit enough to parts-mounting by flow-soldering, namely, solder-flow mounting process (applied mainly to single-sided printed wiring board).
Heating and radiating ultraviolet, in particular, during the production of the printed wiring board tend to vaporize, decompose and polymerize the repellent, and result in shortening a repellant effective period.
To overcome those inconveniences, Japanese Patent Application No. H09-018117 discloses a technique of mixing repellent directly into material of electronics components.
In this material, repellent having a heat decomposition temperature of 250 degrees is used, so that the material withstands the solder-flow mounting process (applied mainly to single-sided printed wiring board) and achieves an initial repelling rate of approx. 95%.
Recently, the market has demanded higher reliability against vermin of various apparatuses such as home-use and business-use telephones, security system, alarm, POS system, intercom with camera, vending machine, portable telephone and PHS base station, public telephone, card reader. As a matter of course, a printed wiring board (PWB) with copper through-holes and a multi-layer PWB mainly used in those apparatuses have been strongly required to employ the material including the repellent.
However, parts-mounting to the foregoing boards uses solder-paste in soldering, i.e., solder-reflow mounting process, so that it takes as long as 30-180 seconds to raise the board surface temperature of 200° C. to the peak temperature of, e.g., 240° C. Thus the repellent in the conventional material decreases (vaporization, decomposition, polymerization) and cannot fully exert its repelling effect (initial repelling rate: approx. 30%). Further, it is very rare that a PWB with copper through-holes or a multi-layer PWB with copper through-holes completes its parts-mounting through only a single solder-reflow process.
To be more specific, on those boards, parts with lead-wires and surface-mounted parts intermingle with each other, so that parts are mounted by solder-reflow and solder-flow, or solder-reflow and again solder-reflow. As such, various combinations of solder-reflow and solder-flow are used in the parts-mounting process. The PWBs and other electronics components undergo severe heating condition throughout the foregoing mounting process, so that substantial amount of the repellent in the conventional board decreases and the repelling effect also decreases (initial repelling rate: 10%).
Therefore, the material cannot be used in the PWB that needs the foregoing mounting process. On top of that, lead-free solder has been widely used to meet the environmental protection movements. In the case of using the lead-free solder in part-mounting, a proper peak temperature of a board ranges 260-280° C. at solder-flow mounting, and 250-270° C. at solder-reflow mounting.
The higher peak temperature needs to set a retention time of the board (raise the board surface temperature of 200° C. to the peak temperature of e.g., 260° C.) 60-240 seconds. Under those severe heating condition, the conventional material including the repellent or the PWB using the material incurs substantial decrease (vaporization, decomposition, polymerization) of the repellent, and the repelling effect completely disappears (initial repelling rate: 0%). As a result, the conventional material including the repellent cannot be used in the parts-mounting that employs lead-free solder.